At a restaurant in most countries anything brought to your table that you didn't order is considered complimentary. A small basket of bread or some antipasto is simply a gesture of generosity, goodwill from the restaurateur. But not in Portugal.
The traditional custom which operates is that anything on your table, known as a couvert, is simply placed there for your convenience.
If you don't want it the onus is on you to ask for it to be removed. So don't be shy about this, there will be no issue with you sending back things you have not specifically ordered.
Usually "courverts" consist of a small dish of olives, bread, salad or even some seafood.
Luckily, if you do find that you have eaten these items it usually only costs about one or two Euro - and they are things you usually wouldn't have minded ordering anyway.
Do be aware however that sometimes this practice can be used to trick travellers in particularly touristy areas in Lisbon - they may try to nakedly exploit the inattention of foreigners.
Here you may find that a simple cheese plate, long forgotten by the end of the meal, will set you back more than $7.50.
If caught in a situation like this there it little you can do but simply grimace and accept this is all part of the experience of traveling.
And finally it's worth a side note that Portuguese restaurants will look at you blank faced should you have the temerity to ask for a "doggie bag".
This is not a traditional custom respected in Portugal; in fact at a formal dinner party it is polite to leave a small portion of your food.
So if you are on a tight budget remember to eat up there may be no taking leftovers back to the hostel!